1. Field of the Invention
The present invention relates to data communication systems and, in particular, to a receiver squelch circuit having a programmable threshold.
2. Discussion of the Prior Art
The basic function of a data communications system is to transmit information over a communications channel from an information source to a destination that is physically separated from the source.
Some communications channels (e.g. twisted pair cables) have inherent characteristics that make it difficult for them to transmit digital data. To permit the transmission of digital data over these channels, it is necessary to convert the digital data at the point of transmission to a form that is compatible with the channel. This is done by utilizing the data to modulate a carrier waveform which is within the bandwidth of the channel. The modulated carrier waveform is then transmitted on the channel and demodulated at the receiver to recover the transmitted data.
While passing through the communication channel, the modulated signal suffers systematic distortion and noise contamination. Thus, a receiver typically includes a squelch circuit for filtering out noise from the received signal.
As shown in FIG. 1, a conventional squelch circuit utilizes specified positive and negative threshold levels V.sub.THP and V.sub.THN, respectively, to establish valid data points in recovering data from the received input signal R.sub.x.
Detection of threshold levels in the input signal R.sub.x is used to identify zero crossings ZC in the signal R.sub.x that represent valid data points D. Zero crossings that do not meet the established threshold level criteria are dismissed as noise.
Conventional squelch circuits, however, have fixed threshold levels. Thus, a particular squelch circuit can be used only in applications having a noise level safely below the circuit's threshold.
It would, therefor, be highly desireable to have available a receiver squelch circuit having a threshold that can be varied based on the measured noise level of the transmission system in which it is to be used.